An optical drive device that records or reproduces information onto or from an optical disk such as a CD (Compact Disc), a DVD, and a BD (Blu-ray disc®) includes an optical pickup. The optical pickup has an outward optical system that generates an optical beam and condenses the optical beam onto a recording surface of the optical disk by an objective lens, and a returning optical system including an optical detector that receives an optical beam reflected from the recording surface of the optical disk.
The optical beam needs to be focused on a center of a track formed on the recording surface of the optical disk. Therefore, the optical drive device performs a control, which is called “tracking servo”, to adjust a deviation of a focus position in a radial direction. This tracking servo is briefly explained below.
FIG. 54(a) shows an end surface of a cross section of a recording surface of an optical disk 11 configured by plural lands L and grooves G, an objective lens 104, and an optical beam (incident light, reflection light (zero-order diffracted light, ± (plus/minus) first-order diffracted light)). As shown in FIG. 54(a), zero-order diffracted light and plus-first-order diffracted light interfere with each other in an area PE1, and zero-order diffracted light and minus-first-order diffracted light interfere with each other in an area PE2. Areas where zero-order diffracted light and ±first-order diffracted light interfere with each other like the areas PE1 and PE2 are called “push-pull area(s)”.
FIG. 55 shows a light receiving surface 101 of an optical detector that receives an optical beam reflected from the recording surface of the optical disk 11. As shown in FIG. 55, zero-order diffracted light reflected from the recording surface of the optical disk 11 forms a spot at the center of the light receiving surface 101. The spot forms various shapes such as a quadrangle and a round shape by various kinds of lenses arranged in an optical path. In this example, a circular spot is drawn.
As shown in FIG. 55, the light receiving surface 101 has a square shape, and is divided into upper and lower areas. As a result of this division, an upper light receiving area 101A receives light of the push-pull area PE1, and a lower light receiving area 101B receives light of the push-pull area PE2.
The optical detector that receives an optical beam outputs a signal having amplitude of a value (a light receiving amount) obtained by performing surface integration of intensity of the optical beam in a light receiving surface, for each light receiving area. An output signal corresponding to a light receiving area X is hereinafter expressed as Ix.
Light intensities in the push-pull areas PE1 and PE2 become values corresponding to a difference between a phase of the zero-order diffracted light and a phase of the ±first-order diffracted light and the intensities of the light. The phase difference and the intensities change depending on unevenness on the recording surface. Therefore, when a focus position of incident light shifts to a radial direction of the optical disk, that is, a direction crossing the track (lateral direction in FIG. 54(a)) (hereinafter, the shift is called “track jump”), the difference between a phase of the zero-order diffracted light and a phase of the ±first-order diffracted light and the intensities of the light change following this shift. The light intensities in the push-pull areas PE1 and PE2 also change. As a result, each of the above output signals also changes.
FIG. 54(b) shows a change of each output signal. As shown in FIG. 54(b), output signals I101A and I101B show changes of mutually opposite phases around a predetermined value a. An added signal of these signals I101A+I101B becomes always a constant value 2a. 
On the other hand, a subtracted signal of the output signals I101A−I101B (hereinafter, the signal is called “push-pull signal PP”) becomes 0, when a focus position of the incident light is at the center of the land L or the groove G. In other cases, this push-pull signal PP becomes a value other than 0. Tracking servo uses a characteristic of this push-pull signal PP. The optical drive device outputs the push-pull signal PP as a tracking error signal TE. The optical drive device adjusts a deviation of the focus position in the radial direction of the optical disk, by controlling a position of the objective lens 104 to set the tracking error signal TE to 0.
Various offsets occur in the tracking error signal TE. Specifically, there occur an offset following a positional shift (a lens shift) of the objective lens due to the tracking servo, and an offset occurring in a boundary (a record boundary) due to a difference between reflectance ratios in an area (a recorded area) in which data is recorded and in an area (an unrecorded area) in which data is not yet recorded. In a multilayered disk, an optical beam (stray light) reflected from a layer other than a layer to be accessed interferes with an optical beam (signal light) reflected from the layer to be accessed, thereby generating an offset. The offset becomes a cause of generating an error in the tracking servo. Therefore, it is required to decrease the offset from the tracking error signal TE.
Japanese Patent Application Laid-open No. H6-176381 discloses in paragraphs 0044 to 0050, a configuration of decreasing offsets due to an optical deviation including the lens shift mentioned above from the tracking error signal TE, by dividing an optical flux of signal light into two, providing an optical detector for each divided light, and by using a difference between the push-pull signals PP obtained for each optical detector as the tracking error signal TE.
Japanese Patent Application Laid-open No. 2005-346882 (abstract) and Japanese Patent Application Laid-open No. 2007-287232 (abstract) disclose a configuration of decreasing offsets due to the lens shift from the tracking error signal TE, by a technique called “differential push-pull method”. The differential push-pull method also uses a difference between push-pull signals PP obtained for each optical detector as the tracking error signal TE, by dividing an optical flux of signal light and by providing an optical detector for each divided light, in a similar manner to a technique disclosed in Japanese Patent Application Laid-open No. H6-176381.
The differential push-pull method is explained in detail. This technique decreases offsets generated in the tracking error signal TE due to a shift (a lens shift) of a position of an objective lens by tracking servo. An optical beam irradiated on the recording surface of the optical disk 11 is passed to a diffraction grating, and is decomposed into zero-order diffracted light and ±first-order diffracted light. These zero-order diffracted light and ±first-order diffracted light are different from the zero-order diffracted light and ±first-order diffracted light described above. To avoid confusion, the zero-order diffracted light, plus-first-order diffracted light, and minus-first-order diffracted light that are decomposed by the diffraction grating are hereinafter called “main beam MB”, “sub beam SB1”, and “sub beam SB2”, respectively. When zero-order diffracted light and ±first-order diffracted light are referenced, these indicate diffracted light generated by diffraction on the recording surface. The main beam MB, the sub beam SB1, and the sub beam SB2 independently generate reflection light having the push-pull areas described above.
FIG. 56 shows light receiving surfaces of an optical detector 100 used to perform tracking servo by using the differential push-pull method. The optical detector 100 receives an optical beam reflected from the recording surface of the optical disk 11, and has three light receiving surfaces 101 to 103 as shown in FIG. 56. Centers of the light receiving surfaces 101 to 103 are arranged to coincide with spot centers of the main beam MB, the sub beam SB1, and the sub beam SB2, respectively. Each spot has various shapes such as a quadrangle and a round shape by various kinds of lenses arranged in an optical path. In this example, a circular spot is drawn.
The light receiving surfaces 101 to 103 have a square shape, and are divided into upper and lower areas. The push-pull area PE1 irradiates light to light receiving areas 101A, 102B, and 103B at the upper side in FIG. 56. The push-pull area PE2 irradiates light to light receiving areas 101B, 102A, and 103A at the lower side in FIG. 56. As explained above, an upper-and-lower relationship of the main beam and the sub beams is reversed.
When the differential push-pull method is not used, the optical drive device uses a main push-pull signal MPP (the same signal as the push-pull signal PP) as the tracking error signal TE. When the differential push-pull method is not used like this, an offset generated in the tracking error signal TE due to the lens shift is cancelled by another certain method. Thereafter, a position of the objective lens 104 is controlled to set the tracking error signal TE to 0. As a result, a deviation of the focus position of the radial direction of the optical disk can be adjusted.
On the other hand, when the differential push-pull method is used, the tracking error signal TE is shown by the following equation (1). In this equation, SPP represents a sub push-pull signal, and is expressed by (I102A+I103A)−(I102B+I103B).
                                                        TE              =                            ⁢                              MPP                -                kSPP                                                                                        =                            ⁢                                                (                                                            I                                              101                        ⁢                        A                                                              -                                          I                                              101                        ⁢                        B                                                                              )                                -                                  k                  ⁢                                      {                                                                  (                                                                              I                                                          102                              ⁢                              A                                                                                +                                                      I                                                          103                              ⁢                              A                                                                                                      )                                            -                                              (                                                                              I                                                          102                              ⁢                              B                                                                                +                                                      I                                                          103                              ⁢                              B                                                                                                      )                                                              }                                                                                                          (        1        )            
Due to the lens shift, an offset occurs in the same direction in the main push-pull signal MPP and the sub push-pull signal SPP. That is, although a spot shifts to the upper and lower directions of FIG. 56 due to the lens shift, a shift direction is the same for the main beam and the sub beams. Therefore, when the spot shifts to the upper side in FIG. 56, for example, the light receiving amount in the light receiving areas 101A, 102A, and 103A increases, and the light receiving amount in the light receiving areas 101B, 102B, and 103B decreases. As a result, the output signal I101A increases, the output signal I101B decreases, and an offset occurs in an increase direction of the main push-pull signal MPP. Similarly, the output signals I102A and I103A increase, the output signals I102B and I103B decrease, and an offset occurs in an increase direction of the sub push-pull signal SPP. Therefore, these changes can be cancelled by the equation (1).
As described above, the upper-and-lower relationship of the push-pull areas is reversed between the main beam and the sub beams. Therefore, phases of the main push-pull signal MPP and the sub push-pull signal SPP are different by 180° from each other. Accordingly, the main push-pull signal MPP and the sub push-pull signal SPP are not cancelled by each other by the equation (1). By determining a constant k in advance to cancel the offset generated in the main push-pull signal MPP and the sub push-pull signal SPP at a lens shift time, tracking servo can be performed by the equation (1).
Japanese Patent Application Laid-open No. 2004-281026 discloses, in paragraph 0111, an example of a technique of performing tracking servo by using the differential push-pull method. This example has an object of removing an offset generated in the tracking error signal TE due to the use of an optical disk having a track positional deviation (formation failure) for every three tracks, and achieves the object by not using a vicinity of the center of each light receiving surface. When a track positional deviation occurs in every three tracks, three tracks can be considered as one cyclical structure, and this cycle becomes three times a track pitch. A diffraction angle of diffracted light from this cyclical structure becomes small by the length of the cycle. Therefore, the diffracted light from the cyclical structure largely depends on a center portion of the beam. Consequently, the above offset can be removed by not using the vicinity of the center of each light receiving surface.
The optical drive device focuses an optical beam on a center of the track, by controlling a position of the objective lens in a radial direction of the optical disk as well as in a direction perpendicular to an optical disk recording surface (for example, see Japanese Patent Application Laid-open No. 2007-328833 (paragraphs 0002 to 0014)).
Position control of the objective lens including control in a direction perpendicular to the optical disk recording surface is explained again collectively including the tracking servo described above.
First, in accessing the optical disk, the optical drive device starts generating three kinds of signals including the tracking error signal TE, a pull-in signal PI, and a focus error signal FE, based on a light receiving amount of an optical beam received by an optical detector. Position control of the objective lens is performed by using these signals.
While a technique of controlling the position of the objective lens is explained below in detail by using each of the above signals, before the explanation, structures of the optical detector and the optical beam are briefly explained.
FIG. 57 is an outline view of an optical detector 110 contained in the optical pickup, viewed from an irradiation direction of the optical beam. X and Y directions shown in FIG. 57 correspond to a tangent direction of the optical disk and a radial direction of the optical disk, respectively.
As shown in FIG. 57, the optical detector 110 includes three light receiving surfaces 111 to 113 that are all quadrate. In these light receiving surfaces, the light receiving surface 111 is divided into four quadrates (light receiving areas 111A to 111D) of the same dimensions. The light receiving areas 112 and 113 are divided into two of upper and lower areas (light receiving areas 112A and 112B and light receiving areas 113A and 113B) having the same dimensions. The optical beam is irradiated to the optical disk in a state of being diffracted in zero-order diffracted light (the main beam MB), plus-first-order diffracted light (the sub beam SB1), and minus-first-order diffracted light (the sub beam SB2), by a diffraction grating (not shown) provided in an outward optical system. The light receiving surfaces 111 to 113 are arranged at positions where the main beam MB, the sub beam SB1, and the sub beam SB2 can be received.
As shown in FIG. 57, each of the beams MB, SB1, and SB2 has the push-pull areas PE1 and PE2 described above. As shown in FIG. 57, a positional relationship of the push-pull area PE1 and the push-pull area PE2 is opposite between the main beam MB and the sub beams SB1 and SB2.
In the tracking error signal TE, a focus position of the optical beam becomes 0 when the focus position of the optical beam is at the center of the track as viewed from above the recording surface, and is other value than 0 in other cases. The optical drive device controls a position of the objective lens to a radial direction of the optical disk, and sets a value of the tracking error signal TE to 0, thereby setting a focus of the optical beam to the center of the track as viewed from above the recording surface (tracking servo).
Generally, a differential push-pull signal DPP expressed by the following equation (2) is used as the tracking error signal TE (the differential push-pull method). In this equation, MPP and SPP represent a main push-pull signal, and sub push-pull signal, respectively, and are expressed by the following equations (3) and (4), respectively. In the equation (2), k represents a positive constant.DPP=MPP−kSPP  (2)MPP=(I111A+I111D)−(I111B+I111C)  (3)SPP=(I112A+I103A)−(I112B+I113B)  (4)
Relative intensities in the push-pull areas PE1 and PE2 shown in FIG. 57 change following a shift of a focus position of the beam incident to the recording surface, when this focus position shifts to a radial direction of the optical disk, that is, a direction crossing the track (track jump). When the focus position of the beam incident to the recording surface is at the center of the track, intensities in the push-pull areas PE1 and PE2 become equal. Therefore, a value of the main push-pull signal MPP becomes 0 when the focus position of the beam incident to the recording surface is at the center of the track, and becomes other value than 0 in other case. This similarly applies to the sub push-pull signal SPP. However, a phase of the sub push-pull signal SPP is different from that of the main push-pull signal MPP by 180°, and these phases are opposite. As described above, a positional relationship of the push-pull area PE1 and the push-pull area PE2 is opposite between the main beam MB and the sub beams SB1 and SB2.
Tracking servo can be also performed by using only the main push-pull signal MPP. That is, by controlling a position of the objective lens to a radial direction of the optical disk, thereby setting the value of the main push-pull signal MPP to 0, in principle, a focus of the optical beam can be set to the center of the track viewed from above the recording surface.
Nonetheless, the differential push-pull signal DPP shown in the equation (2) is used, to decrease the influence of an offset (hereinafter, “lens shift offset”) generated in the main push-pull signal MPP following a shift of the objective lens. This lens shift offset is briefly explained below.
Each spot shown in FIG. 57 shifts a Y direction (a signal light radial direction) to the same direction, following the shift of the objective lens. For example, when each spot shifts to the upper side in FIG. 57, the light receiving amount in the light receiving areas 111A, 111D, 112A, and 113A increases, and the light receiving amount in the light receiving areas 111B, 111C, 112B, and 113B decreases. As a result, the output signals I111A and I111D increase, the output signals I111B and I111C decrease, and an offset in an increase direction occurs in the main push-pull signal MPP. Similarly, the output signals I112A and I113A increase, the output signals I112B and I113B decrease, and an offset in an increase direction also occurs in the sub push-pull signal SPP.
In the differential push-pull signal DPP shown in the equation (2), a sign of the sub push-pull signal SPP is minus. Therefore, by properly determining the positive constant k, the shift offset described above generated in each of the main push-pull signal MPP and the sub push-pull signal SPP can be cancelled. Consequently, in the differential push-pull signal DPP, the influence of lens shift offsets can be decreased.
An optimum value of the constant k when there is no influence of stray light is a ratio (hereinafter, “beam intensity ratio”) of the intensity of the main beam MB to the intensity of a sum of the intensities of the sub beams SB1 and SB2. However, in a multilayered optical disk, there is an influence of stray light from the recording surface different from the focus position. Therefore, this ratio does not necessarily become an optimum value. Accordingly, a value of the constant k when a multilayered optical disk is used is determined to cancel lens shift offsets generated in the main push-pull signal MPP and the sub push-pull signal SPP, respectively.
The pull-in signal PI takes a relatively large value when a focus position of the optical beam is near the recording layer of the optical disk, and takes a relatively small value when the focus position is not near the recording layer. Specifically, the pull-in signal PI is expressed by a total of output signals in all light receiving areas within the light receiving surface 111, as shown in the following equation (5). However, the pull-in signal PI is normally output in a state of limiting a band by passing this signal through a low-pass filter. The band limit is performed to remove an RF signal and noise.PI=I111A+I111B+I111C+I111D  (5)
The optical drive device compares a value of the pull-in signal PI with a predetermined threshold value. By detecting a portion having a value larger than the threshold value, the optical drive device detects that a focus position of the optical beam is brought near to the recording layer. This detection is called “layer recognition”. This control is performed to recognize that a vicinity of a specific layer (a layer to be accessed) is focused among plural recording layers, in parallel with focus servo described later.
The focus error signal FE becomes 0 when a focus position of the optical beam is on the recording layer of the optical disk. Specifically, the focus error signal FE is expressed by the following equation (6). The optical drive device controls a position of the objective lens to a direction perpendicular to the recording surface of the optical disk. By setting a value of the focus error signal FE to 0, the optical drive device focuses the optical beam on the recording layer. This control is called “focus servo”.FE=(I111A+I111C)−(I111B+I111D)  (6)
A cylindrical lens (not shown) is arranged in the returning optical system. Spots formed on the light receiving surface 111 by the main beam MB become in an oblong shape slender in an inclined direction as shown by spots MB1 and MB2 in FIG. 57, when the optical beam is not focused on the recording layer. In this case, the value of the focus error signal FE becomes a value other than 0. On the other hand, when the optical beam is focused on the recording layer, a spot becomes a round shape as shown by a spot MB0 in FIG. 57. In this case, a value of the focus error signal FE becomes 0. Focus servo utilizes a characteristic of this focus error signal FE.